In recent years, the automobiles have widely adopted a steering apparatus configured so that a steering column constructed to include two components, i.e., an inner column and an outer column, becomes collapsible to absorb a secondary collision energy in order to relieve an impact exerted on a driver upon the secondary collision with the steering wheel in case of an accident. In this type of steering apparatus, the inner column and the outer column relatively slide on each other in an axial direction, whereby the steering wheel moves toward the front of a vehicle, and an energy absorbing means provided between the inner column and the outer column absorbs the secondary collision energy.
For example, in the steering apparatus described in Patent document 1, the outer column disposed on a lower side is secured to a car body through a tilt bracket and a tilt pivot, and a clamping mechanism clamps the outer column to thereby hold the inner column disposed on an upper side. In this steering apparatus, an ingression-enabled quantity of the inner column into the outer column is set larger than a telescopic adjustable quantity, and the inner column moves to the front side of the vehicle while resisting a clamping friction force of the clamping mechanism upon the secondary collision.
What is required of this steering apparatus is to smoothly absorb the secondary collision energy generated when a driver having a light weight collides with the steering wheel. To fulfill this requirement, it is considered to reduce a clamping force of the tilt/telescopic adjustment mechanism; however, with the clamping force being decreased, the force of holding the inner column decreases, resulting in a backlash that is easy to occur at a fitting portion between the inner column and the outer column. Such being the case, according to Patent document 1, a low friction material treatment instanced by coating is applied over an outer peripheral surface of the inner column or an inner peripheral surface of the outer column, thereby reducing the clamping friction force without decreasing the clamping force.
In a telescopic type steering apparatus, the steering shaft is generally configured to include an inner shaft and an outer shaft, which are spline-joined within the steering column in order to render a transfer of a steering torque and a telescopic adjustment compatible with each other. In this case, resin coating is often applied over one of two splines, i.e., a male spline and a female spline, for preventing emission of backlash noises due to a minute gap between the male spline and the female spline.